1. Field of the Invention
The present invention relates to an electrophoretic display device and a driving method thereof. More specifically, the present invention relates to an electrophoretic display device and a driving method thereof, with which afterimages and ghosting are not generated and inverted images (negative images) are not displayed when updating images.
2. Description of the Related Art
As a display device capable of conducting an action of “reading” without stress, electronic paper display devices referred to as electronic books or electronic newspapers are being developed. The electronic paper display device of such type is required to be thin, light in weight, hard to be broken, and low in power consumption. In order to satisfy those demands, a reflective display medium having a display memory characteristic is generally considered as advantageous. An electrophoretic display device, an electronic powder type element, a cholesteric liquid crystal element, and the like are known as such display media. Recently, an electrophoretic display device using two or more kinds of charged particles has drawn an attention. Hereinafter, a display device using an electrophoretic display element is simply referred to as an “electrophoretic display device”. However, it is to be noted that the concept of the “electrophoretic display device” includes the type of display device using an element that provides display by migration of charged particles, such as an electronic powder type element. The basic principle of the electrophoretic display device is that a liquid cell (electrophoretic layer) containing charged particles is sandwiched by transparent electrodes, and the reflectance of the display surface changes by migration of the charged particles caused by adding a voltage. Recently, an active matrix driving type electrophoretic display device using a film-type electrophoretic layer on which a great number of microcapsules filled with charged particles and a solvent and using a TFT glass substrate has been put into practical use. This electrophoretic display device is constituted by stacking a TFT glass substrate, an electrophoretic display element film (electrophoretic layer), and a counter substrate in this order. In the TFT glass substrate, TFTs as a great number of switching elements arranged in matrix, pixel electrodes, gate lines, and data lines connected to each of the TFTs, respectively, are provided. Further, the electrophoretic display element film is formed by filling microcapsules of about 40 μm in a polymer binder. In the solvent inside the microcapsules, two kinds of nanoparticles charged in plus and minus, i.e., white pigment such as titanium oxide particles charged minus and black pigment such as carbon particles charged plus are confined in a dispersedly floating manner. Further, a counter electrode (also referred to as a common electrode) for giving a reference potential is formed on the counter substrate.
The operations of the above-described active matrix drive type electrophoretic display device are executed by migration of the white pigment and the black pigment vertically through applying voltages corresponding to pixel data between the pixel electrode and the counter electrode. That is, when the positive voltage is applied to the pixel electrode, the white pigment charged minus is gathered to the pixel electrode while the black pigment charged plus is gathered to the counter electrode. Thus, assuming that the counter electrode side is the display surface, the area (pixel) on the display screen corresponding to the pixel electrode to which the positive voltage is applied turns out as black display. In the meantime, when the negative voltage is applied to the pixel electrode, the black pigment is gathered to the pixel electrode and the white pigment is gathered to the counter electrode. Thus, the corresponding area (pixel) on the display screen turns out as white display. Further, the migration amount of the charged particles can be changed through changing the voltage applying time and the extent of the voltage, so that it is possible to provide halftone (gray) display (e.g., Japanese Unexamined Patent Publication 2009-276763 (Patent Document 1)). As described, it is possible to display characters, images, and the like through controlling the voltages to be applied for each pixel electrode.
However, when voltages corresponding to a next image are simply applied to the pixel electrodes at the time of updating the displayed image with the electrophoretic display device, the history of the previous image affects the next image, which is visually recognized as an afterimage. Therefore, it is being tried to cancel the history of the previous image by, for example, providing the so-called reset period in which white/black display is repeated in all the pixels on the display screen once, white-black inverted image is displayed in a next image, etc. Therefore, the voltages applied to the pixel electrodes at the time of update include not only the voltages corresponding to prescribed display colors but also voltages in the reset period, and change for the period (time) during the update of images. A series of voltages applied to the pixel electrodes from the start to the end of the update of the image are referred to as a voltage waveform at the time of updating the image.
Further, it is known to cause deterioration in the display quality such as having afterimages, ghosting, and the like with the electrophoretic display device when DC (Direct Current) components are accumulated (residual electric charges are generated) by repeating the update of the images. As a driving method which suppresses the accumulation of the DC components, there is a driving method which zeros the total amount (time-integrated voltage value) of the voltages applied to the pixel electrode. Japanese Patent Application Publication 2008-509449 (Patent Document 2) and Japanese Patent Application Publication 2007-512571 (Patent Document 3) disclose examples of the driving method which zeros the total DC components. FIG. 61 shows examples of the waveforms disclosed in Patent Document 2. Each graph of FIG. 61 shows the waveform of the voltage given to the pixel electrodes when updating the image, in which the lateral axis is the time (seconds) and the longitudinal axis is the applied voltage (V). Expression of [R1 R2] in FIG. 61 is a symbol while defining the display before updating the image, i.e., the initial state, is R2 and the display after updating the image, i.e., the final state, is R1. That is, FIG. 61 shows the total of sixteen voltage waveforms of transitions in four gradations including gray display.
Specific examples of the expression [R1, R2] are shown in the followings.    [1 1]: Transition from black (gray level 1) to black (gray level 1)    [3 1]: Transition from black (gray level 1) to light gray (gray level 3)    [4 1]: Transition from black (gray level 1) to white (gray level 4)
Details of each waveform will be described by referring to the [1 4] waveform as an example. The [1 4] waveform is constituted with: a first reset pulse of +15 V and 0.5 seconds, which drives the pixels to black; a second reset pulse of −15 V and 0.5 seconds, which drives the pixels to white; and a set pulse of +15 V and 0.5 seconds, which drives the pixels to black. The [1 4] waveform achieves transition from the gray level 4 (white) to gray level 1 (black), i.e., achieves update of an image. Referring to FIG. 61, the total DC of the voltages applied to the pixels in a single-time update of the image is zero in the waveforms of [1 1], [2 2], [3 3], and [4 4]. In the meantime, as in the case of the [1 4] waveform, for example, there is a waveform having deviation in DC component with a single-time update of the image. Those waveforms are so described in Patent Document 2 that the total DC becomes zero by conducting the update of images for a plurality of times.
Further, FIG. 63 shows examples of the waveforms disclosed in Patent Document 3. FIG. 63 shows the typical waveforms used at the time updating the images from white to white, from light gray to light gray, from dark gray to dark gray, and from black to black, in which R1, R2 are the reset pulses, GD is a gray scale drive pulse, and ED is a polar drive pulse (pulse that drives the optical state of pixels from one of the polar optical state to the other polar optical state). Patent Document 3 refers to FIG. 63 and describes that the net DC of each gray scale transition (state between the intermediate gray optical state and the intermediate gray optical state such as the state between light gray and light gray or between dark gray and dark gray) at the time of updating the image, i.e., the product of the voltage and the time at each pulse, is zero. Further, it is also described that the net DC becomes the minimum for each polar transition (e.g., between white and white, between black and black).
However, with the electrophoretic display device driving method which prevents the conventionally generated after images and ghosting and increases the display quality, an inverted image of the displayed image and an inverted image of the image to be displayed next are displayed in the reset period at the time of updating the image. This gives a sense of discomfort to the user.
For example, the inverted image displayed in the reset period will be described by using the Driving Example shown in FIG. 61 and the display example shown in FIGS. 62A and 62B of Patent Document 2. FIGS. 62A and 62B illustrate the changes in the display screen when the image is updated with the waveforms of FIG. 61 in the display matrix of 6×8 pixels. FIG. 62A shows the expression of the gray level, the initial image before updating the image, and the final image after updating the image. As described above, the expression of [R1 R2] shows the transition between the gradations used in Patent Document 2. Note that black is expressed as B (gray level 1), white is expressed as W (gray level 4), and [4 1] shows the transition from black to white.
FIG. 62B shows a midway state during update of the image by the waveform of FIG. 61, the display screen after 0.5 seconds passed from the point of applying the first reset pulse, and the display screen after 1.0 seconds passed from the point of applying the second reset pulse. As shown in FIG. 62B, the display screen after 1.0 seconds passed from the point of applying the second reset pulse becomes an almost white-black inverted image of the final image. The change speed of the display state of the electrophoretic display device such as the transition time from white to black, for example, is more gradual compared to the case of the liquid crystal display device and the like. Thus, the white-black inverted image after 1.0 seconds shown in FIG. 62B including the changes before and after is sufficiently recognized by the eyes of human beings. Therefore, the user is to visually recognize the inverted image of the image displayed next every time the image is updated, thereby giving a sense of discomfort to the user.
Further, in the Driving Example shown in FIG. 63 of Patent Document 3, the display state of almost an white-black inverted image of the final image is displayed at the end of R1 (reset pulse). This occurs because the polarity of R1 is the opposite polarity of the pulse to be applied at last for forming the final image shown in FIG. 63. As described above, the transition time of the particles of the electrophoretic display device is generally gradual. Thus, the user also comes to visually recognize the white-black inverted image also in the Driving Example of FIG. 63, thereby giving a sense of discomfort to the user.
As described above, one of the reasons why the inverted image is recognized is that the migration time of the particles is slow. Further, the reset pulse is required for increasing the display quality, so that the image according to the voltage applied in the reset period is to be visually recognized. However, to zero the total DC component for increasing the display quality is to execute the transition reversed from the final transition, i.e., to apply the voltage that is of inverted polarity of the voltage applied in the set period in the reset period for the same length of time. That is, when a waveform of zero total DC is simply devised, an inverted image is generated as a result. As will be described later, the inventors of the present invention have done eager studies to find the driving method which can achieve zero total DC and generate no inverted image, and have achieved the present invention.
Further, in the Driving Example of Patent Document 2, the polarities of the set pulse and the reset pulse vary depending on the display gradations (gray levels) as in the waveforms shown in FIG. 61. Therefore, when employed to the active matrix drive, an instantly large driving capacity is required for the drive circuit that supplies the voltage to the pixels. For example, it is assumed that two pixels such as a pixel making transition from black to black [1 1] and a pixel making transition from white to white [4 4] are in a relation of being adjacent to each other by being connected via a same voltage supply line and connected sequentially to selection lines for selecting switching modules. In that case, a radical change in the potential of the voltage supply line is required at the time of supplying the reset pulse, i.e., −15 V for the [1 1] pixel and +15 V for the next [4 4] pixel. Further, at the time of supplying the set pulse, a radical change in the potential of the voltage supply line is also required at the time of supplying the reset pulse, i.e., +15 V for the [1 1] pixel and −15 V for the next [4 4] pixel. It is necessary for the drive circuit to satisfy this demand. However, when a large scaled screen and high definition thereof are more advanced, the driving capacity to be required becomes larger as well. Thus, in order to correspond to the large scaled screen and the high definition thereof, a waveform that does not require a radical change in the potential, i.e., drive with suppressed power consumption, is required.
The present invention is designed in view of the above-described issues. An exemplary object of the present invention is to provide an electrophoretic display device and a driving method thereof, with which the DC component can be made zero in the driving waveforms in total, afterimages and ghosting are not generated, and inverted image is not displayed at the time of updating the image. Further, an exemplary object of the present invention is to provide an electrophoretic display device and a driving method thereof, which require low power consumption by employing a waveform that does not require a radical change in the potential.